MINOR: connection: implement alpn registration of muxes

Selecting a mux based on ALPN and the proxy mode will quickly become a
pain. This commit provides new functions to register/lookup a mux based
on the ALPN string and the proxy mode to make this easier. Given that
we're not supposed to support a wide range of muxes, the lookup should
not have any measurable performance impact.
diff --git a/src/connection.c b/src/connection.c
index 129c741..43ec328 100644
--- a/src/connection.c
+++ b/src/connection.c
@@ -30,6 +30,11 @@
 struct pool_head *pool2_connection;
 struct xprt_ops *registered_xprt[XPRT_ENTRIES] = { NULL, };
 
+/* List head of all known muxes for ALPN */
+struct alpn_mux_list alpn_mux_list = {
+        .list = LIST_HEAD_INIT(alpn_mux_list.list)
+};
+
 /* perform minimal intializations, report 0 in case of error, 1 if OK. */
 int init_connection()
 {